1. Field of the Invention
This invention relates to heat exchangers and, more particularly, to a leak-safe Hydrogen/Air heat exchanger wherein liquid hydrogen is used as primary coolant fluid for chilling the high speed air stream of a hypersonic vehicle propulsion system. Modern hypersonic aircraft, to operate at high Mach numbers with liquid hydrogen fuel, are being designed utilizing ram air, chilled to a saturated vapor, or condensed to a liquid, as a source of oxidizer for combustion in the propulsion system. Propulsion systems of this nature are generally referred to as Liquid Air Cycle Engines or LACE, and will be referred to in this disclosure as such.
Other systems that enrich air with oxygen prior to storage or combustion, usually chill the air to a saturated vapor condition before admitting it to an enrichment device. These have been termed Air Collection and Enrichment, or ACE systems. This disclosure assumes that the air is enriched with oxygen, thereby producing a stream of "waste" nitrogen which emerges from the enrichment device as saturated vapor. It has been shown that enriching air with oxygen can significantly improve vehicle mission performance (Ref. U.S. Pat. No. 3,756,024).
Air is approximately 20 percent oxygen and 80 percent nitrogen. It becomes saturated and begins to condense at about 180 degrees Rankine (depending on the pressure), and the saturated waste nitrogen is about 20 degrees colder (again depending on the nitrogen pressure). To obtain maximum performance of this ACE system, it is important to use the cooling capacity of the waste nitrogen as well as that of the hydrogen fuel to chill as much air as possible. In this invention, the waste nitrogen also serves as a safety barrier between the hydrogen and air in the main precooler heat exchanger.
Liquid hydrogen is also used as a refrigerant in the enrichment device before its vapors enter the heat exchanger, subject of this invention, a few degrees colder than the saturated waste nitrogen vapor. There is no possibility of freezing nitrogen or air in the cold end of this heat exchanger.
However, at high flight speeds, in excess of Mach 3.5, the incoming air is above the auto ignition temperature for hydrogen and air (about 1000 degrees Fahrenheit), so that a direct leak between the hydrogen and air streams could cause a catastrophic fire or explosion. Furthermore, at any speed, hydrogen leaking into the air stream will adversely affect the performance of the enrichment device. This invention maintains the benefits of liquid hydrogen to provide refrigeration for the enrichment of air with oxygen and to liquefy the product oxygen while eliminating single failure mixing of leaked hydrogen with the supply air stream.
In the precooler heat exchanger, coaxial tubes are utilized, with cold hydrogen in the central one and inert nitrogen gas in the annulus between them. The inert gas then chills the heat exchanger surfaces contacted by the ram air. Ram air is chilled to its saturated vapor condition, the oxygen extracted for storage or combustion, cold nitrogen for cooling, and the cycle repeated. This process thus uses both the refrigerant capacity and chemical inactivity of saturated nitrogen vapor, an otherwise "waste product" of the liquid air cycle enrichment process, to high advantage, in this invention.
Any leak in the central duct system, mixes hydrogen with inert nitrogen. Any leak in the outer duct system mixes nitrogen and air, no combustion resulting from either type failure.
While the preferred embodiment of this invention is presented as usage in ACE systems with nitrogen gas as the inert fluid, the coaxial tube arrangement presented here is applicable to a variety of systems wherein a primary temperature control fluid, be it for heating or cooling of a working fluid, is reactive with the working fluid. Such systems include those using "liquid sodium/water" heaters in nuclear reactors and for many processes in the chemical industry where highly reactive elements are most efficiently processed with one as temperature controller for the other.
Description of the Prior Art
ACE systems are discussed in U.S. Pat. No. 3,756,024 to A. Gay et al., defining a special arrangement for ACE heat exchangers. Cold nitrogen, in '024 is used as coolant for the ram air in addition to hydrogen in the heat exchangers of a main cooler section. Hydrogen leaks in the '024 main cooler section still pose problems of control and explosive dangers should they be near, or migrate to areas near, high temperature ram air. The inert fluid isolation system of this invention is not present in the engine of '024 but would comprise a major improvement if incorporated into that invention.
A liquid air engine of U.S. Pat. No. 3,775,977, to C. Builder, et al., also utilizes precoolers for incoming air but does not use the combination of cold hydrogen and nitrogen isolation of this invention.